Airplanes flying at altitude operate in reduced pressure portions of the atmosphere. For this reason, most commercial airplanes employ a circulation system that maintains at least a minimum pressure in the cabin for passenger comfort. This cabin pressure must be maintained while continuing to allow air to be exchanged between the cabin and the outside environment, thereby assuring a sufficient level of fresh air and oxygen for passengers. In all, the air circulation system of an airplane must assure both a proper air flow rate and a proper pressure in the cabin. To do so, at least some airplanes include an air inflow valve through which air can be introduced at a specified rate and a forward and rear adjustable exit valves.
Aside from the above requirements, typical airplanes are equipped with multiple environmental control systems that serve to condition the air circulated through the plane. These systems include temperature control systems, cabin air distribution systems, electronic equipment cooling systems, cargo heat and air conditioning systems, lavatory and galley exhaust systems, nitrogen generation systems, and moisture control systems, amongst others. In some cases, the environmental control systems negatively affect the air passing through those systems. For example, some systems add undesirable amounts of thermal energy to the air; other systems release particles into the air that cause unwanted odors. It is desirable that any such affected air remain separated from the passenger areas of the plane, and a typical strategy for accomplishing this objective is to cause inflow of air into the cabin to be equal to the flow rate of air through the environmental control systems. Forward and rear exit valves are configured to open and close in proportion to one another such that the proportion of air exiting the respective valves is similar to the proportion of air flowing in systems located, respectively, in the forward and rear portions of the plane. A typical distribution of valve and system flow rates is 25% in the front and 75% in the rear (at steady state). Thus, the rear exit valve is typically opened so as to permit three times the flow rate permitted by the forward exit valve. In this way, the inflow of air serves to entrain the affected air from the environmental control systems and causes that air to be expelled from the cabin via the exit valves before it is re-circulated in the cabin and reaches the passengers.
In more recent times, airplanes have incorporated some environmental control systems that vary temporally in the amount of air they utilize and affect. This configuration is generally shown in FIGS. 1a and 1b, in which a pressurized compartment 10 includes an inflow valve 12, forward and rear exit valves 14a-b, and forward and rear environmental control systems 16a-b. As shown in the figures, forward system 16a varies between a “Mode 1” state, in which the system has a steady state flow capacity of 500 cubic feet per minute (cfm) and a “Mode 2” state, in which the system has a flow capacity of 1500 cfm. At the same time, the rear system 16b maintains a constant flow capacity of 1500 cfm. As such, the total flow of air into and out of the pressurized compartment is 2000 cfm when the forward system is in Mode 1 and 3000 cfm when the forward system is in Mode 2.
As shown, air enters the pressurized compartment from the inflow valve 12 in amount equal to the aggregate capacity of the forward and rear systems 16a-b. Because the air flow requirements vary over time, the capacity of the inflow valve and exit valves must be varied correspondingly. However, in most conventional aircraft, the forward and rear valves vary together, such that the proportion of air flowing through each one is constant. In FIGS. 1a and 1b, the proportion of the flow exiting the forward exit valve is 25%, 75% going through the rear exit valve (a typical distribution, as discussed earlier). When the forward system is in Mode 1, this proportion matches the proportion of flows through the forward and rear systems. In that case, the air flowing through the forward system exits from the forward exit valve, and the same is true for the rear system and exit valve. However, when the forward system is in Mode 2, the proportion of flows through the forward and rear systems does not match that of the forward and rear exit valves. In that case, some of the air flowing through the forward system is “re-circulated” through the pressurized compartment until it ultimately exits at the rear exit valve. This can be disadvantageous in that the re-circulated air may be malodorous or excessively hot, and during the re-circulation, such air may pass through the cabin and come in contact with occupants of the cabin. This strategy of adjusting the forward and rear exit valves in a set proportion is therefore ineffective for expelling affected air before it reaches passengers. A new strategy is therefore desired, in which passengers are isolated from negatively impacted portions of the internal environment even as the operation of systems that varies with time.